In a display apparatus of a non-self emitting type, a backlight section is employed as a light source for displaying a picture. An example of the display apparatus of a non-self emitting type is a liquid crystal display apparatus.
The backlight section of such a liquid crystal display apparatus can be a cold cathode fluorescent tube or an LED (Light Emitting Diode).
In the case of a cold cathode fluorescent tube employed as a backlight section, a power-supply section of the display apparatus includes an inverter circuit for generating an alternative current voltage used for driving the backlight section.
As shown in FIG. 7, typically, such an inverter circuit inputs a direct current power supplied by a main power-supply circuit employed in a liquid crystal display apparatus, and generates an alternative current voltage.
In the liquid crystal display apparatus shown in the figure, first of all, a rectification/smoothing circuit 101 inputs a commercial alternative current power-supply AC and generates a direct current voltage. Then, a DC-DC converter connected to the rear stage of the rectification/smoothing circuit 101 to serve as a main power-supply circuit 102 carries out a DC-DC power conversion process on the direct current voltage generated by the rectification/smoothing circuit 101 to generate a direct current power-supply voltage at a stabilized predetermined level. Typically, the main power-supply circuit 102 employs an isolation transformer serving as a direct current isolator between the primary and secondary sides. That is to say, the primary side, which is the commercial alternative current power-supply side, inputs a direct current voltage whereas the secondary side outputs a direct current power-supply voltage.
As shown in the figure, the direct current power-supply voltage output by the secondary side of the main power-supply circuit 102 is supplied to a load 103, which is driven by the direct current power-supply voltage to operate. In addition, also as shown in the figure, the direct current power-supply voltage output by the secondary side of the main power-supply circuit 102 branches, being supplied to an inverter circuit 104.
The inverter circuit 104 carries out a DC-AC power conversion process on the received direct current power-supply voltage to generate an alternative current voltage, which is supplied to a backlight section 105. The backlight section 105 is driven by the alternative current voltage to emit light.
In this case, the main power-supply circuit 102 is provided with a switching converter on the primary side and a rectification/smoothing circuit on the secondary side. In this configuration, a switching output obtained on the primary side is rectified and smoothed on the secondary side to generate a direct current voltage serving as the power-supply voltage. Thus, as shown in the figure, the inverter circuit 104 receives the direct current power-supply generated by the secondary side of the main power-supply circuit 102.
Then, as described above, the inverter circuit 104 carries out a DC-AC power conversion process on the received direct current power-supply to generate an alternative current voltage, which is supplied to the backlight section 105 as a driving voltage.
On the other hand, FIG. 8 is a diagram showing the configuration of a liquid crystal display apparatus having a backlight section composed of LEDs. It is to be noted that sections shown in FIG. 8 as sections identical with their respective counterparts explained earlier by referring to FIG. 7 are denoted by the same reference numerals as the counterparts.
In the case of a backlight section 110 composed of LEDs as shown in the figure, chopper regulators 109 are provided on the secondary side as a circuit for driving the backlight section 110. In the case of the typical configuration shown in FIG. 8, a plurality of chopper regulators 109a, 109b and 109c are connected in parallel to each other to a plurality of LEDs, which form the backlight section 110.
To be more specific, the chopper regulators 109a, 109b and 109c are each connected to a circuit including a plurality of LEDs connected to each other in series. The chopper regulators 109 receive a direct current voltage from the secondary side of the main power-supply circuit 102 and carries out a DC-DC power conversion process on the direct current voltage. Then, a direct current voltage obtained as a result of the DC-DC power conversion process is stabilized in a voltage stabilization process according to a result of detecting the level of a current flowing through each of the LEDs. The stabilized voltage is applied to the LEDs as a driving voltage to emit light from each of the LEDs.
The chopper regulators 109 are connected in parallel to each other in this configuration to keep up with, for example, a case in which the number of LEDS is relatively large to form a big-size screen of the liquid crystal display apparatus and a case requiring a relatively high level as a direct current level for producing necessary high luminance. That is to say, if only one chopper regulator 109 is used for driving a plurality of LED series connection circuits in a configuration including a large number of LEDs to be driven and a configuration requiring a large current as described above, the size of the circuit of the single chopper regulator 109 itself is big and, in order to solve this problem, a plurality of chopper regulators 109 is connected in parallel to each other.
If a backlight section 110 including LEDs as described above is used, the chopper regulators 109 receive a direct current voltage from the main power-supply circuit 102 and carries out a DC-DC power conversion process on the direct current voltage. Then, a direct current voltage obtained as a result of the DC-DC power conversion process is used as a direct current power-supply of the backlight section 110.
It is to be noted that Japanese Patent Laid-open No. Hei 2-79182 discloses a technology relating to an inverter circuit provided for a case in which a fluorescent tube is used as a light source of a display apparatus.
In addition, Japanese Patent Laid-open No. 2002-244103 discloses a technology relating to a chopper regulator provided for a case in which LEDs are used as a light source.
By the way, as shown in FIG. 7 previously, the inverter circuit 104 is provided at the stage behind the main power-supply circuit 102. Thus, a power supplied to the inverter circuit 104 is a result of a power conversion process carried out in the main power-supply circuit 102. Then, in order to generate an alternative current voltage for driving the backlight section 105, the inverter circuit 104 again carries out a power conversion process.
That is to say, in the conventional configuration shown in FIG. 7, in order to drive the backlight section 105, two power conversion processes are carried out in the main power-supply circuit 102 and the inverter circuit 104 respectively.
Also in the case of the configuration shown in FIG. 8, in order to drive the backlight section 110, two DC-DC power conversion processes are carried out in the main power-supply circuit 102 and the chopper regulator 109 respectively.
By carrying out a plurality of power conversion processes as described above, the power conversion efficiency decreases, raising the power loss.
Particularly, in recent years, a technological revolution in the liquid crystal display field increases the size of the display screen increasing thereby the power consumption for driving the backlight so that the power consumption of the set as a whole rises. For a screen size of 40 inches, for example, the power consumption of the set as a whole is about 250 W for some cases. In the case of display apparatus manufactured in recent years as apparatus with large screens, the power loss reaches a relatively high level.
In addition, in such cases, with the display screen increased and the power consumptions of the inverter circuit 104 and the chopper regulator 109 rising, in consequence, it is necessary to keep up with the large power of the main power-supply circuit 102. That is to say, since the inverter circuit 104 and the chopper regulator 109 are provided at the stages behind the main power-supply circuit 102, as the power consumptions of the inverter circuit 104 and the chopper regulator 109 increase, the power of the main power-supply circuit 102 also rises by an increase that should be kept up with.
Thus, in the conventional configurations shown in FIGS. 7 and 8, as the size of the display screen increases, the size of the main power-supply circuit 102 also rises so that the cost to manufacture the circuit of the main power-supply circuit 102 becomes higher as well.
In addition, by letting the large power of the main power-supply circuit 102 be kept up with as described above, the amount of heat dissipated due to an actual power loss increases. In order to keep up with the increasing amount of dissipated heat, it is necessary to reserve a space large enough to serve as a countermeasure of the dissipated heat or to set a countermeasure by providing a cooling fan.
If a space is provided as a countermeasure of dissipated heat, however, the space certainly entails a large size of the apparatus. In addition, if a cooling fan is provided, the operation sound of the fan will serve as a source of discomfort suffered by the user.